1 Dec
2011
1 Dec
'11
1:23 p.m.
Brent Hilpert wrote:
In time, it was quantized when it was recorded and
retains that
quantisation when read
(note I said discrete time slots relative to each
other).
No, it doesn't. The drive and media shift the peaks around, such that
they only have an approximate relation to the "discrete time slots
relative to each other" in which they were recorded.
While there is some room for a little looseness in
phrasing when
dealing with (signals from)
a physical medium, it's still a stretch to call
the disk signal
'analog in the time domain', esp.
when the phrase has a firmer meaning in other
applications.
It's not at all a stretch. The flux transitions definitely do NOT have
the same timing as the bits that were written, due to both magnetic and
mechanical effects. The timing is changed enough that it is actually
quite difficult to recover the original timing. If the flux
transitions, and thus the read data pulse leading edges on the
interface, were still time-quantized, this would be trivial.
The fact that there were aftermarket improved data separators sold for
the TRS-80 Model 1 Expansion Interface, because of the unreliability of
the built-in data separator of the FD1771, is one indication of just how
non-time-quantized the read data is. A great deal of research went into
developing data separators that could restore the time quantization with
reasonable reliability; there are many published papers on this topic,
and many patents.
Eric